translation - Untranslated passages in translated works

I constantly find in translations of classic works untranslated passages. In English translations of Russian novels I find dialogue or narrative in French - as if the English-reading public, who clearly need the Russian translated, are expected to read French. Latin phrases are almost never translated. Do you people think we're all Latin scholars, do you think we don't care to know what's being said, do you just like leaving us in the dark, or are you too lazy to convey those passages?